Electronic thermostats have many features including energy saving operations and wireless communications for remote access. Typically, these electronic thermostats use 24 VAC or battery power to operate.
Thermostats with wireless RF communications can draw quite a bit of power, and when power is lost, the primary function of temperature control fails. During winter months, a home without heat for an extended period of time can lead to freezing water pipes that eventually burst, costing expensive repairs and loss of valuables.
For 24 VAC power type thermostats with rechargeable battery backups, there is still a chance that a battery will have drained after some period of lost AC. There is no failsafe mechanism to keep gas heat running. For battery power, the same problem can occur if a homeowner forgets to periodically replace current batteries with a fresh set, and there is no failsafe mechanism to keep a house temperature within normal limits.